This disclosure relates to guns that propel projectiles using compressed gas as a propellant. More particularly, it relates to an improved gun barrel for use in combination with a gas powered projectile gun firing soft or pliable ammunition such as paint balls or pepper balls. Paint balls have a liquid center covered by a thin plastic or gelatin membrane that maintains the paint ball in an approximately spherical shape. Pepper balls have a powder filled center covered by a thin hard plastic shell that is nevertheless flexible. Both types are called “soft projectiles” in this application. This application incorporates by reference the disclosure of U.S. Patent Publication No. 2005/0247295, titled “Barrel and ball sizer for paint-ball gun,” published Nov. 10, 2005.